Self-Propelled Civil Engineering Machine And Method Of Controlling A Civil Engineering Machine

ABSTRACT

A civil engineering machine and a method of controlling the machine are based on the position of at least one reference point which is relevant to the control of the civil engineering machine being changed, as the civil engineering machine moves, as a function of a relative position of the at least one reference point relative to a desired path of travel.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a self-propelled civil engineering machine, andin particular a slipform paver, and to a method of controlling aself-propelled civil engineering machine.

2. Description of the Prior Art

There are a variety of known types of self-propelled civil engineeringmachine. Such machines include for example known slipform pavers or roadmilling machines. A distinguishing feature of self-propelled civilengineering machines is that they have a working unit which is arrangedon a chassis of the machine and which has work-doing means for producingstructures on a piece of ground or for changing the piece of ground.

On known slipform pavers, the working unit comprises an arrangement formoulding flowable material, and in particular concrete, which will alsobe referred to below as a concrete mould. Structures of differentconfigurations, such as crash barriers or traffic islands for example,can be produced with the concrete mould. A slipform paver is describedin, for example, EP 1 103 659 81.

The working unit of known road milling machines is a milling arrangementwhich has a milling drum fitted with milling tools with which materialcan be milled off the ground over a preset working width.

Known civil engineering machines also have a drive unit having drivingmeans to enable translatory or rotational movements to be performed bythe civil engineering machine on the ground, and a control andcalculating unit to control the drive unit in such a way that the civilengineering machine performs the translatory and/or rotational movementson the ground.

To produce structures on the ground or to change the ground, an attemptis made in the case of self-propelled civil engineering machines toachieve largely automatic control of the civil engineering machinewithout the need for any interventions worth mentioning by the driver ofthe machine. When the civil engineering machine is being controlledautomatically, the drive unit of the civil engineering machine iscontrolled in such a way that, starting from a preset starting point atwhich the civil engineering machine is in a preset position and at apreset orientation on the ground, a reference point on the civilengineering machine is moved for a preset distance or path of travel orat a preset spacing therefrom, i.e. along a line equidistant from thedistance or path of travel, in order to produce a structure or to changethe ground. The preset distance or path of travel is defined by straightlines and/or curves in this case.

A known method of controlling self-propelled civil engineering machinespresupposes the use of a string line by which the desired distance orpath of travel or a line equidistant from the desired distance or pathof travel is preset. It is also known for self-propelled civilengineering machines to be controlled by the use of a global navigationsatellite system (GNSS). A road milling machine whose drive unit iscontrolled by a string line is known for example from U.S. Pat. No.4,041,623. U.S. Pat. No. 5,988,936 describes a slipform paver having asystem for controlling the drive unit by the use of a string line. Inthe case of both these civil engineering machines, the spacing between areference point on the civil engineering machine and the string line isdetermined by sensing members which detect the string line.

The slipform paver which is known from U.S. Pat. No. 5,988,936 has asensing member which is at the front in the direction of operation and asensing member which is at the rear in that direction and a centralsensing member which is arranged between the front and rear sensingmembers. All the sensing members lie on a common axis which extendsparallel to the longitudinal axis of the slipform paver. The sensingmembers define reference points on the civil engineering machine.

When the slipform paver is moving along a straight section of thedesired distance or path of travel, both the sensing member which is atthe front in the direction of operation and the sensing member which isat the rear in that direction are in use and the drive unit is thuscontrolled both as a function of the distance measured between theassociated front reference point on the civil engineering machine andthe string line and as a function of the distance measured between theassociated rear reference point and the string line. The drive unit isso controlled in this case that the spacing from the string line isequal to a desired value. Control of this kind has proved satisfactoryin practice.

When the slipform paver is moving along a curved section on the otherhand, the control of the drive unit takes place as a function only ofthe spacing which is measured between the associated central referencepoint on the civil engineering machine and the string line. The frontand rear sensing members are not active in this case.

At the transition from a straight section to a curved section or viceversa, a changeover is therefore needed between the various sensingmembers. At the entry into the curve, a changeover is made from thefront and rear sensing members to the central sensing member and at theexit from the curve a changeover is made back from the central sensingmember to the front and rear ones. A changeover to various differentsensing members proves to be a disadvantage simply because thechangeover means that the process is not a continuous one. It is also adisadvantage that, when travel through a curve is controlled by thecentral sensing member, the position of the sensing member depends onthe radius of the curve.

If however the slipform paver is to change its direction of operation atthe transition from a straight section to a curved section, thenproblems arise in practice when the drive unit is controlled in theknown way where provision is made for a changeover from the front andrear sensing members to the central sensing member at the entry into thecurve. What is a particular problem for the control system in this caseis entry into tight curves, such for example as when concrete componentsin the form of traffic islands are being produced.

It has also been found that the transition from the straight section ofthe desired distance or path of travel to the curved section is aparticular problem for the control of the civil engineering machine,because the position of the discontinuity is not precisely known. It istrue that in practice the driver of the machine can specify that theentry into the curve is to start. However, the exact position of thediscontinuity is difficult for the driver of the machine to estimate.The position of the discontinuity cannot be determined solely bymonitoring the spacing between the front reference point and the stringline, which increases at the entry into the curve, simply because thecontrol system is intended to correct any difference between thereference point and the string line, i.e. to keep the spacing to zero.Even if the changeover should take place at exactly the right point intime, what results in practice will be an abrupt steering commandbecause it is most unlikely that the central reference point will be atexactly the same spacing from the string line as the front and/or rearreference point at the time of the changeover.

SUMMARY OF THE INVENTION

The object underlying the invention is to improve the automatic controlof a self-propelled civil engineering machine particularly when thecivil engineering machine is moving from a substantially straightsection of the preset distance or path of travel onto a curved sectionthereof.

The way in which this object is achieved in accordance with theinvention is by virtue of the features of the independent claims. Thedependent claims relate to advantageous embodiments of the invention.

The civil engineering machine according to the invention and the methodaccording to the invention are based on the position of at least onereference point which is relevant to the control of the drive unit ofthe civil engineering machine being changed, as the civil engineeringmachine moves, as a function of the line followed by the desireddistance or path of travel in a co-ordinate system referred to the civilengineering machine. The position of the reference point is preferablychanged continuously. It is however also possible for the position ofthe reference point to be changed in individual steps. All that iscrucial is for the reference point to assume a plurality of differentpositions.

Before the transition from a substantially straight section of thedesired distance or path of travel to a curved section of the desireddistance or path of travel, i.e. before the discontinuity, the positionof the at least one reference point is shifted from a position which isat the front in the direction of operation to a position which is at therear in the direction of operation, whereas after the transition from asubstantially straight section of the desired distance or path of travelto a curved section, i.e. after the discontinuity, the position of theat least one reference point is shifted from a position which is at therear in the direction of operation to a position which is at the frontin the direction of operation.

What is meant in this case by a shift in the position of the referencepoint is that what is taken for the purpose of controlling the driveunit is not the position of a fixed reference point in the co-ordinatesystem referred to the civil engineering machine but that the referencepoint for determining position moves at the entry into the curve.

This reference point may be an “imaginary” point on the civilengineering machine. The desired distance or path of travel may also bean “imaginary” distance or path. This will be the case when the desireddistance or path of travel is laid down in a co-ordinate system and theposition of the reference point is determined by reference to thedesired distance or path of travel using a global navigation satellitesystem (GNSS). If however the desired distance or path of travel is laiddown on the ground by a string line and if the spacing of a referencepoint from a string line is measured by means of a spacing sensor, thenthe reference point is laid down by the location of the sensor on thecivil engineering machine.

What is meant by a substantially straight section may in practice alsobe a section which has a radius. In practice, a section whose radius isfor example more than 10 m may be considered to be a substantiallystraight section.

In a preferred embodiment of the invention, the control and calculatingunit of the civil engineering machine has means for determining thedeviation from a desired distance or path of travel of a reference pointon the civil engineering machine which is at the front in the directionof operation and of a reference point thereon which is at the rear inthe direction of operation, the control and calculating unit being sodesigned that, in a first mode of control, the drive unit is controlledas a function of the deviation of the reference points at the front andrear in the direction of operation when the civil engineering machine ismoving along a substantially straight section of the desired distance orpath of travel, the front and rear reference points thus moving alongthe desired distance or path of travel or along the desired distance orpath of travel at a preset spacing therefrom, i.e. along a lineequidistant from the desired distance or path of travel.

The control and calculating unit is so designed that the reference pointwhich is at the front in the direction of operation is shifted from aposition which is at the front in the direction of operation to aposition which is at the rear in the direction of operation before thetransition from a substantially straight section of the desired distanceor path of travel to a curved section of the desired distance or path oftravel. After the transition from a substantially straight section ofthe desired distance or path of travel to a curved section of thedesired distance or path of travel, the reference point which is at thefront in the direction of operation is shifted from the position whichis at the rear in the direction of operation back to the position whichis at the front in the direction of operation.

The control and calculating unit is preferably so designed that, in asecond mode of control, the drive unit is controlled as a function ofthe position of the reference point which is at the rear in thedirection of operation when the civil engineering machine is movingalong a curved section of the desired distance or path of travel, therear reference point thus moving along the desired distance or path oftravel or along the desired distance or path of travel at a spacingtherefrom. Alternatively, the machine might also be controlled along thecurved section from two reference points situated next to and very closeto one another, one of which would then be situated a short distance infront of the outlet from the mould and the other would be situated ashort distance behind it.

The drive unit having the driving means to enable the civil engineeringmachine to perform translatory and/or rotational movements on the groundpreferably has front wheels or running-gear units and rear wheels orrunning-gear units and a steering arrangement for steering the frontwheels or running-gear units and/or the rear wheels or running-gearunits.

Basically, entry into a curve can be initiated by the driver of themachine by his operating for example a switch, push-button or the like.It is however also possible for entry into a curve to be detected by theuse of a global navigation satellite system (GNSS), the desired distanceor path of travel being laid down in a co-ordinate system and theposition of the discontinuity thus being known.

An embodiment of the invention will be explained in detail in whatfollows by reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view from the side of an embodiment of a slipform paver.

FIG. 2 is a view from the side of an embodiment of a road millingmachine.

FIG. 3.1 is a highly simplified schematic view of a slipform paver in afirst position where the slipform paver is moving along a straightsection of a preset desired distance or path of travel.

FIG. 3.2 shows the slipform paver in a second position where theslipform paver is moving along the straight section towards a firstdiscontinuity in the preset distance or path of travel.

FIG. 3.3 shows the slipform paver in a third position where the slipformpaver is continuing to move along the straight section towards the firstdiscontinuity in the preset distance or path of travel.

FIG. 3.4 shows the slipform paver in a fourth position where theslipform paver is situated at the first discontinuity in the presetdistance or path of travel.

FIG. 3.5 shows the slipform paver in a fifth position where the slipformpaver is situated at the first discontinuity.

FIG. 3.6 shows the slipform paver in a sixth position where the slipformpaver is moving along a curved section of the preset distance or path oftravel.

FIG. 3.7 shows the slipform paver in a seventh position where theslipform paver is moving along the curved section of the preset distanceor path of travel towards a second discontinuity.

FIG. 3.8 shows the slipform paver in an eighth position where theslipform paver is continuing to move along the curved section of thepreset distance or path of travel towards the second discontinuity.

FIG. 3.9 shows the slipform paver in a ninth position where the slipformpaver is situated at the second discontinuity.

FIG. 3.10 shows the slipform paver in a tenth position where theslipform paver is situated at the second discontinuity.

FIG. 3.11 shows the slipform paver in an eleventh position after thetransition from the curved section of the preset distance or path oftravel to the straight section thereof.

DETAILED DESCRIPTION

FIG. 1 is a view from the side of a slip form paver which serves as anexample of a self-propelled civil engineering machine. Because slipformpavers as such are part of the prior art, all that will be describedhere are those components of the civil engineering machine which arematerial to the invention.

The slipform paver 1 has a chassis 2 which is carried by running gear 3.The running gear 3 has two front and two rear track-laying running-gearunits 4A, 4B which are fastened to front and rear lifting columns 5A,5B. The direction of operation (direction of travel) of the slipformpaver is indicated by an arrow A.

The track-laying running-gear units 4A, 4B and the lifting columns 5A,5B are part of a drive unit to enable the civil engineering machine toperform translatory and/or rotational movements on the ground. Byraising and lowering the lifting columns 5A, 5B, the chassis 2 of themachine can be moved relative to the ground to adjust its height andinclination. The civil engineering machine can be moved backwards andforwards with the steerable track-laying running-gear units 4A, 4B. Thecivil engineering machine thus has three degrees of freedom intranslation and three in rotation.

The slipform paver 1 has an arrangement 6 for moulding concrete which isonly indicated and which will be referred to below as a concrete mould.The concrete mould is part of a working unit which has work-doing meansfor producing a structure 7 of a preset shape on the ground.

FIG. 2 is a view from the side of a self-propelled road milling machinewhich serves as a further example of a civil engineering machine. Theroad milling machine too has, once again, a chassis 9 which is carriedby running gear 10. The running gear 10 once again has front and reartrack-laying running-gear units 12A, 12B which are fastened to front andrear lifting columns 13A, 13B. The road milling machine has a workingunit 14 which has work-doing means to change the ground. This is amilling arrangement 14 having a milling drum 14A fitted with millingtools.

FIGS. 3.1 to 3.11 show various positions of a civil engineering machine,which is only shown in a highly simplified form, when entering andleaving a curve. The present embodiment is a slipform paver which ismerely indicated. It has a chassis 15, a drive unit 16 having front andrear track-laying running-gear units 17A, 17B, a steering arrangementfor steering the front and rear track-laying running-gear units 17A,17B, and a concrete mould 19.

The slipform paver is going to produce, as a structure, a traffic islandin the form of a “cigar”. For this purpose, the slipform paver has tomove along a preset distance or path of travel which will be referred toin what follows as the desired distance or path of travel 20. FIGS. 3.1to 3.11 show only part of the desired distance or path of travel bywhich the geometrical shape of the cigar is defined.

The desired distance or path of travel 20 has a first straight section20A which merges into a radiused section 20B covering 180°, which isagain followed by a straight section 20C. In the present embodiment, theline followed by the desired distance or path of travel is laid down ina co-ordinate system (X, Y) which is independent of the movement of thecivil engineering machine. As well as the fixed co-ordinate system (X,Y), what is also shown in FIGS. 3.1 to 3.11 is a co-ordinate system (x,y) referred to the civil engineering machine.

To control the drive unit 16, the civil engineering machine has acontrol and calculating unit 23 which is merely indicated. The controland calculating unit 23 controls the drive unit 16 in such a way thatthe civil engineering machine performs on the ground the translatoryand/or rotational movements required to enable it to produce thestructure 22 or change the ground when the civil engineering machinemoves along the preset desired distance or path of travel. The controland calculating unit 23 comprises all the components required to carryout calculating operations and to generate control signals for the driveunit 16. It may be one self-contained sub-assembly or may comprise aplurality of separate sub-assemblies which may not only be arranged onthe civil engineering machine but some or all of which may also bearranged on the ground near to the civil engineering machine.

In the present embodiment, the control and calculating unit 23 has aglobal navigation satellite system (GNSS) 24 which comprises a firstGNSS receiver 24A and a second GNSS receiver 24B which are arranged indifferent positions on the civil engineering machine. As well as the twoGNSS receivers, the global navigation satellite system (GNSS) may alsohave, on the ground, a reference station (not shown) for generatingcorrecting signals. Using the two GNSS receivers, the GNSS system 24determines data which gives the positions of the GNSS receivers in theco-ordinate system (X, Y). As well as this, the control and calculatingunit may also have a programmable logic control system which is alsoreferred to as a PLC system.

From the positions of the two GNSS receivers 24A, 24B and the knowngeometry of the civil engineering machine, the control and calculatingunit 23 calculates the position of a reference point 25 on the civilengineering machine which is at the front in the direction of operationand the position of a reference point 26 on the machine which is at therear in the direction of operation. The two reference points 25, 26 lieon a straight line which extends parallel to the longitudinal axis ofthe civil engineering machine. The rear reference point 26 is situatedin this case in line with that edge of the concrete mould 19 which is onthe inside and at the rear in the direction of travel. This edgecorresponds to the outer boundary of the structure 22 to be produced.The two reference points are arranged at an original spacing I.

The control and calculating unit 23 also has means for determining datadefining the line followed by the desired distance or path of travel 20.Using a virtual design model, the line followed by the desired distanceor path of travel is preset in the co-ordinate system (X, Y). Thisdesign model may be entered manually or may be read into a memory 23Abelonging to the control and calculating unit 23 from a data carrier.

As well as this, the control and calculating unit 23 also has means fordetermining the deviation from the desired distance or path of travel 20of the positions of the reference point 25 which is at the front in thedirection of operation and of the reference point 26 which is at therear in the direction of operation.

The control and calculating unit 23 controls the drive unit, i.e. thefront and rear track-laying running-gear units 17A, 17B, as a functionof the spacing between the reference point and the desired distance orpath of travel. The control and calculating unit provides for thispurpose two different modes of control.

In the first mode of control, the drive unit 16 is controlled as afunction of the spacing between the rear reference point 26 and thedesired distance or path of travel 20 and as a function of the spacingbetween the front reference point 25 and the desired distance or path oftravel. The control of the drive unit takes place in such a way that,this spacing of both the rear and the front reference points correspondsto a preset value during an advancing movement of the civil engineering,i.e. the civil engineering machine moves along the desired distance orpath of travel at a preset spacing therefrom. The pivoted position ofthe rear track-laying running-gear units 17B is controlled in this caseas a function of the deviation of the rear reference point 26 from thedesired distance or path of travel 20 and the pivoted position of thefront track-laying running-gear units 17A is controlled as a function ofthe deviation of the front reference point 25 from the desired distanceor path of travel 20. The deviations of the reference points from thedesired distance or path of travel are calculated by the control andcalculating unit using the GPS system 24.

In the second mode of control on the other hand, the drive unit 16 iscontrolled as a function of the deviation only between the rearreference point 26 and the desired distance or path of travel 20. Thecontrol of the front track-laying running-gear units 17A takes place insuch a way that the spacing of the rear reference point 26 on the civilengineering machine corresponds to a preset value during an advancingmovement of the civil engineering machine, i.e. the rear reference pointmoves along the desired distance or path of travel at a preset spacingtherefrom.

FIGS. 3.1 to 3.5 show the movement of the civil engineering machine asit advances along the straight section 20A of the desired distance orpath of travel 20. During this movement, the control and calculatingunit 23 presets the first mode of control and the two reference points25, 26 are thus “active”. The two reference points lie one behind theother in this case at a preset spacing 1 on an axis parallel to thelongitudinal axis of the machine.

As it travels along the straight section 20A, the civil engineeringmachine approaches the first discontinuity 30, i.e. the point at whichthe straight section 20A merges into the curved section 20B. During thistravel, the control and calculating unit 23 continuously determines notonly the deviation of the reference points from the desired distance orpath of travel but also the distance along the travel which thereference points have covered. The distance covered will be referred toin what follows as the stationing. Because the original spacing 1 of thereference points is known, all that needs to be determined is thedistance along the travel covered by one of the reference points,because the distance covered along the travel by the other referencepoint can then be calculated.

The reference point 25 which is at the front in the direction ofoperation is now moving towards the discontinuity 30. As it does so, thecontrol and calculating unit 13 determines whether the front referencepoint is still on the straight section 20A or whether it is already onthe curved section on which the position of the front reference point iscompared with the stored design model. Consequently, what is availableis not only the deviation and stationing but also data on the curvatureof the desired distance or path of travel, i.e. data which specifieswhether the reference point is on or next to a straight or curvedsection of the desired distance or path of travel. All the data iswritten continuously to the memory 23A of the control and calculatingunit 23.

At the point in time at which the reference point 25 which is at thefront in the direction of operation reaches the discontinuity 30, theposition of the front reference point is changed. The control andcalculating unit generates a control signal at this point in timebecause the front reference point “detects” the beginning of the curve.The front reference point 25 is then shifted backwards along thestraight line, in the co-ordinate system (x, y) referred to the machine,in the opposite direction to the direction of operation, (FIG. 3.2 andFIG. 3.3), until the front reference point is at a preset spacing fromthe rear reference point on the straight line or, preferably, until thefront reference point is on the rear reference point (FIG. 3.4). As thisis done, the spacing 1 between the two reference points is reduced bythe amount by which the stationing increases. It is noted that in FIGS.3.2-3.4 the actual spacing between the points 25 and 26 is changing andis less than the original spacing I which is illustrated for comparison.The shifting of the front reference point preferably takes placecontinuously. The control signal is preferably generated by the controland calculating unit when the reference point is exactly on thediscontinuity. Basically however, it is also possible for the controlsignal to be generated not when the reference point is exactly on thediscontinuity but in the region of the transition between thesubstantially straight section (20A) and the curved section (20B), i.e.a short distance before it reaches the discontinuity or a short distanceafter it does so. In practice, the control signal is generated a shortdistance before the reference point reaches the discontinuity, therebyinitiating the movement of the steering, and this movement of thesteering is completed a short distance after the reference point reachesthe discontinuity.

FIGS. 3.4 and 3.5 show that the front track-laying running gear units17A and the rear track-laying running-gear units 17B are turned to thesteering angle precisely when the front and rear reference points 25, 26are on the discontinuity 30. In practice however, the process of turningto the steering angle is initiated when the front reference point isstill a preset distance before reaching the discontinuity. Similarly,the process of turning to the steering angle is not completed until apreset distance after the front reference point reaches thediscontinuity.

During the turning to the steering angle, the control and calculatingunit 23 makes the changeover from the first mode of control to thesecond. This changeover may however equally well be made manually by thedriver of the machine.

If the control and calculating unit 23 has preset the second mode ofcontrol, in which only the rear reference point 26 is “active”, thefront track-laying running-gear units 17A are controlled only as afunction of the spacing of the rear reference point 26 from the desireddistance or path of travel 20. In the course of this, the positions ofthe front and rear track-laying running-gear units meet the knowncondition for Ackermann steering, something which is indicated by dashedlines in the drawings.

If the rear track-laying running-gear units are situated at the pointwhere the concrete mould is situated, the said rear track-layingrunning-gear units may remain steered in the straight-ahead position.Otherwise, the track-laying running-gear units are set to a theoreticalor calculated steering angle which is not changed. This steering angleshould meeting the condition for Ackermann steering.

FIGS. 3.6 to 3.9 show how the rear reference point 26 moves along thecurved section 20B of the desired distance or path of travel 20 at apreset spacing therefrom. When this happens, steering is only by thefront track-laying running-gear units 17A while no further change ismade in the position of the rear track-laying running-gear units 17B.

When what was previously the rear reference point 26, which may becongruent with what was previously the front reference point 25, hasreached the discontinuity, the control and calculating unit 23 againgenerates a control signal, after which the front reference point 25 isagain shifted forward in the direction of operation.

The shifting of the front reference point 25 takes place until thespacing between the two points again corresponds to the original spacing1. Consequently, the front reference point 25, which is not “active”,moves ahead of the rear reference point 26 which is “active”. The frontreference point 25 is shown as an asterisk designated as 25′ because itis not “active”.

FIG. 3.8 shows the civil engineering machine during its movement alongthe curved section of the preset distance or path of travel. When therear reference point 26 on the civil engineering machine is on thediscontinuity 31 (FIG. 3.9), the front and rear track-layingrunning-gear units 17A, 17B are re-positioned for straight-ahead travel(FIG. 3.10). However, in a similar way to what is done on entry into acurve, this steering process is already initiated when leaving a curvewhen there is still a preset distance of travel before the rearreference point 26 reaches the discontinuity. Similarly, thetrack-laying running-gear units are not positioned for straight-aheadtravel until the rear reference point is a preset distance of travelpast the discontinuity.

The control and calculating unit 23 thereupon again presets the firstmode of control, and control thus again takes place as a function of thedeviation of the two reference points from the desired distance or pathof travel. The civil engineering machine is now moving again along astraight section 30B in the same way as it was before entering thecurve.

The shifting of the front reference point 25 at the transition from astraight section to a curved section enables exact guidance of the civilengineering machine along the desired distance or path of travel to beachieved.

An alternative embodiment of civil engineering machine makes provisionfor use to be made not of a global navigation satellite system (GNSS)but of a string line. This embodiment differs from the embodimentemploying the GNSS system only in that respective sensors (not shown)are provided at the front and rear reference points to measure thespacing from a string line (not shown) rather than the spacing from thevirtual desired distance or path of travel. The string line then extendsalong the solid line (equidistant line) in the interior of thestructure. The locations of the sensors are thus identical with thelocations of the reference points. The spacing sensors may havemechanical sensing members or may be ultrasonic sensors which operatewithout physical contact. Sensors of these kinds are known in the priorart. The sensor which is at the rear in the direction of operation maybe fastened to the chassis of the machine in a fixed position while thefront sensor may be guided on a rail on the chassis of the machine to bedisplaceable in the longitudinal direction. The displacement of thefront sensor may be carried out with a drive (not shown) which may forexample be an electric-motor-driven spindle drive. Hence, what takesplace in the alternative embodiment is a shift not of the frontreference point but of the spacing sensor itself, what is done being notto calculate the spacing from a desired distance or path of traveldefined by co-ordinates in a co-ordinate system but to measure thespacing from a string line which extends along the desired distance orpath of travel. The above-mentioned advantages are obtained in bothembodiments, and this is done by shifting the reference point or byshifting the sensor situated at a reference point.

1-19. (canceled)
 20. A self-propelled civil engineering machine,comprising: a chassis; a working unit arranged on the chassis andconfigured to produce structures on a piece of ground or to change thepiece of ground; a drive unit configured to move the chassis and workingunit in translatory and/or rotational movements on the piece of ground;at least one reference point defined on the civil engineering machine;at least one sensor configured to detect a position of the at least onereference point relative to the piece of ground; and a controllerconfigured to receive input signals from the at least one sensor and tosend control signals to the drive unit, the controller being configuredto determine a deviation of the at least one reference point from adesired path of travel and to control the drive unit as a function ofthe deviation, so that the at least one reference point moves along thedesired path of travel or at preset spacing from the desired path oftravel, the controller being configured such that as the civilengineering machine moves, the position of the at least one referencepoint relative to the civil engineering machine can be changed as afunction of the position of the at least one reference point relative tothe desired path of travel.
 21. The civil engineering machine of claim20, wherein: the controller is configured such that before a transitionfrom a substantially straight section of the desired path of travel to acurved section of the desired path of travel, the position of the atleast one reference point can be shifted relative to the civilengineering machine rearward opposite to a direction of travel of thecivil engineering machine.
 22. The civil engineering machine of claim20, wherein: the at least one reference point includes a front referencepoint and a rear reference point; and the controller is configured suchthat before or when the front reference point reaches a transition froma substantially straight section of the desired path of travel to acurved section of the desired path of travel, the position of the frontreference point is shifted rearward relative to the civil engineeringmachine toward the rear reference point.
 23. The civil engineeringmachine of claim 22, wherein: the controller is configured such thatwhen the rear reference point reaches the transition, the frontreference point is substantially coincident with the rear referencepoint, and the front reference point is inactivated while the rearreference point traverses the curved section of the desired path oftravel.
 24. The civil engineering machine of claim 23, wherein: thecontroller is configured such that as the rear reference point traversesthe curved section of the desired path of travel the position of thefront reference point is shifted forward, relative to the civilengineering machine away from the rear reference point.
 25. The civilengineering machine of claim 22, wherein: the controller is configuredsuch that when the rear reference point reaches the transition, thefront reference point is at a preset spacing from the rear referencepoint.
 26. The civil engineering machine of claim 25, wherein: thecontroller is configured such that before or when the rear referencepoint reaches a second transition from the curved section to anothersubstantially straight section of the desired path of travel, theposition of the front reference point relative to the civil engineeringmachine is shifted forward relative to the civil engineering machine toan original spacing between the front and rear reference points.
 27. Thecivil engineering machine of claim 20, wherein: the drive unit includesone or more front wheels or running-gear units and one or more rearwheels or running-gear units; the at least one reference point includesa front reference point and a rear reference point; and the controlleris configured such that: in a first mode of control a position of thefront wheels or running-gear units and of the rear wheels orrunning-gear units is varied as a function of a deviation of the frontreference point from the desired path of travel and of a deviation ofthe rear reference point from the desired path of travel; and in asecond mode of control the position of the front wheels or running gearunits is varied as a function of the deviation of the rear referencepoint from the desired path of travel.
 28. A method of controlling adrive unit of a self-propelled civil engineering machine, the methodcomprising: (a) defining a defined position of at least one referencepoint on the civil engineering machine; (b) defining a desired path oftravel of the at least one reference point relative to a ground surface,the desired path of travel including at least one substantially straightportion and at least one curved portion; (c) determining a deviation ofthe at least one reference point from the desired path of travel; (d)controlling the drive unit as a function of the deviation such that theat least one reference point on the civil engineering machine movesalong the desired path of travel; and (e) changing the defined positionof the at least one reference point on the civil engineering machine asa function of a relative position of that at least one reference pointrelative to the desired path of travel.
 29. The method of claim 28,wherein: step (e) further comprises, before the relative position of theat least one reference point relative to the desired path of travelreaches a transition from the substantially straight portion to thecurved portion, shifting the defined position of the at least onereference point rearward relative to the civil engineering machine. 30.The method of claim 28, wherein: in step (a), the at least one referencepoint includes a front reference point and a rear reference point; andstep (e) further comprises, before or when the front reference pointreaches a transition from the substantially straight section of thedesired path of travel to the curved section of the desired path oftravel, shifting the defined position of the front reference pointrearward toward the rear reference point.
 31. The method of claim 30,wherein: step (e) further comprises, continuing to shift the definedposition of the front reference point rearward so that the frontreference point is substantially coincident with the rear referencepoint by the time the rear reference point reaches the transition. 32.The method of claim 31, wherein: step (e) further comprises, shiftingthe front reference point forward as the rear reference point traversesthe curved section of the desired path of travel.
 33. The method ofclaim 30, wherein: step (e) further comprises, continuing to shift thedefined position of the front reference point rearward so that the frontreference point is at a preset spacing from the rear reference point bythe time the rear reference point reaches the transition.
 34. The methodof claim 33, wherein: step (e) further comprises, before or when therear reference point reaches a second transition from the curved sectionto another substantially straight section of the desired path of travel,shifting the defined position of the front reference point forwardrelative to the civil engineering machine to an original spacing betweenthe front and rear reference points.
 35. The method of claim 28,wherein: step (a) further comprises defining defined positions of afront reference point and a rear reference point; step (d) furthercomprises controlling positions of front and rear wheels or running gearunits of the drive unit; and step (d) further comprises: (d)(1) in afirst mode of control, varying position of the front and rear wheels orrunning-gear units as a function of a deviation of the front referencepoint from the desired path of travel and a deviation of the rearreference point from the desired path of travel; and (d)(2) in a secondmode of control, varying the position of the front wheels orrunning-gear units as a function of the deviation of the rear referencepoint from the desired path of travel.